Documents often have adherent transparent overlays to provide protection against dirt, moisture, and general wear and tear. A typical protective transparent overlay has a plastic film bearing an aggressive adhesive layer by which it can be permanently adhered to the face of a document. A transparent overlay which does not obscure underlying information is disclosed in U.S. Pat. No. 3,801,183 (Sevelin et al.). The patent discloses legend-containing sheet materials which have retroreflective legend and retroreflective background areas that are substantially indistinguishable under ordinary diffuse light viewing conditions but are clearly distinguishable under retroreflective viewing conditions. Attempts to tamper with information over which the overlay has been applied result in readily detectable disruption of the overlay.
Subsequent to U.S. Pat. No. 3,801,183, a number of patents have issued disclosing other transparent overlays, each of which can be imaged with a pattern or legend that is readily noticeable only when viewed retroreflectively and can be adhesively bonded to a document without obscuring the face of the document. See, for example, U.S. Pat. No. 4,099,838 (Cook et al.) which discloses an overlay wherein the retroreflection from the background and image areas of the pattern is of contrasting colors. See also U.S. Patent No. 4,688,894 (Hockert) and U.S. Pat. No. 4,691,993 (Porter et al.), which disclose transparent overlays that function similarly as those of U.S. Pat. No. 3,801,183 while having the added capability of permitting an authenticating message to be formed in the overlay after it has been adhesively bonded to a document.
U.S. Pat. No. 5,169,707 (Faykish et al.) discloses a transparent overlay having a primary legend which has a readily detectable appearance under ordinary diffuse lighting conditions to provide a first level of screening or verification that can be carried out simply and conveniently without special equipment. A second level of verification can also be provided by the primary legend under retroreflective viewing conditions.
The transparent protective overlays disclosed in the above-cited patents each comprise a monolayer of glass microspheres that makes the overlay difficult to counterfeit but somewhat expensive to make.
U.S. Pat. No. 4,869,946 (Clay) discloses a security card comprising a transparent upper layer having narrow parallel lenses on its outer surface and an image-containing substrate on its inner surface. The two layers form a lenticular system by which images on the substrate are selectively viewable depending upon the angle from which the card is viewed.
U.S. Pat. No. 3,503,315 (de Montebello) discloses a lenticular plate or sheet comprising adjacent lenslets with spherical convex surfaces on both sides. The sheet is used for the production and display of a picture in panoramic stereoscopic relief.
U.S. Pat. No. 2,951,419 (Lemelson) discloses retroreflective devices which appear, to an observer moving with a light source, to flash on and off, scintillate, or change optical composition. In one embodiment, the device comprises a transparent sheet and a retroreflective sheeting which meet at an interface. At the interface, there is provided two groups of adjacent strips having different color or light reflectivity. At its front surface, the transparent sheet is formed with a number of lens-like formations. An observer moving with a light source sees alternating patterns of color as discrete bands of light from the moving light source enter the device in such a manner that the bands shift from one group of strips to the other group of strips and vice versa.
U.S. Pat. No. 4,576,850 (Martens) concerns shaped plastic articles made by replicating a microstructure-bearing surface with an ultraviolet-curable organic oligomeric composition.
A number of other patents also concern shaped plastic articles made by replicating a microstructure-bearing surface. For example, see U.S. Pat. No. 3,689,346 (Rowland); U.S. Pat. No. 4,414,316 (Conley); U.S. Pat. No. 4,420,502 (Conley); and U.S. Pat. No. 4,374,077 (Kerfeld).